Method of making a nickel-chromium diffusion alloy



ticular merit.

Unite States Patent 01 METHOD OF MAKING A NICKEL-CHROMIUM DIFFUSION ALLOY Glenn R. Schaer, Columbus, Ohio, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application January 31, 1957, Serial No. 637,594

2 Claims. ((31. 204-37) The present invention relates to the 'electrodeposition of metal coatings on .base metals and more particularly to nickel and chromium electrodeposits for use at elevated temperatures.

Many base metals and alloys having high strengths at elevated temperatures do not have satisfactory oxidation resistance. For example, molybdenum oxidizes rapidly at temperatures above 650 .C. The domestic abundance of molybdenum coupled with the fact that it is an extremely hard metal with a melting point that ranks fifth highest among the metallic elements hasplaced increased demands upon finding a suitable coating for molybdenum that will prevent rapid oxidation.

Various methods for applying protective coatings on molybdenum and other refractory metals are ,known, and, of these, several electroplated coatings have par- A protective electroplated metal at a high temperature must not only resist oxidation from the environment in which the coating will be placed but must also prevent deterioration of the base metal caused by the environment. The oxidation resistance of most commercially usable electrodeposited metals is dependent upon the formation of an adherent and impervious .oxide coating. In addition to oxidation resistance, .the electroplate and its scale must withstand impact and abrasion, stress corrosion, and creep. If the electroplate diifnses rapidly with the base metal, the coating life might be shortened.

Discontinuities in the coating reduce its ability to protect the base metal. Thus the density and structure of electrodeposits are important properties. "The coating must prevent the diffusion of gas that could oxidizethe base metal and also prevent the diflEusion of the :base metal through the coating. in addition, the electrodeposit must be strong and ductile enough to resist stress 9 introduced by impact or by force resulting from-any difierence in expansion between the base metal and .the electrodeposit while they are being heated or cooled. Furthermore, the bond strength of the electroplal to :the base metal and any alloy formed by diffusion of the base metal and the electroplate must also withstand such stresses.

Among the electrodeposits that were often considered previously for high temperature protection, nickel and chromium enjoyed prominent positions. Of these two,

chromium resists oxidation in air better and .is "harder Also chromium difv than nickel at high temperatures. fuses more slowly with other metals than nickel .and, furthermore, chromium can be deposited on many refractory materials, particularly molybdenum, with abetter adherence than is possible by plating nickel. However, chromium ruptures at high temperatures with relatively light loads and the oxide scale on chromium is more brittle and less adherent than the oxide scale on nickel, sochromium plate is less satisfactory than nickel for cyclic applications.

As both nickel and chromium each have some desirable 2,859,158 Patented Nov. 4, 1958 properties required for a satisfactory electroplate coating, it is expedient to combine these two metals to take advantage of their desirable features. One known method of combining nickel and chromium is to electroplate alternate layers of these metals on a base material and then alternately heat treat and cold roll the'material until the alloying metals are completely diffused into surface portions of the base metal. methodof combining nickel and chromium has two inherent disadvantages. First of all, the alternate heat treating'and cold rolling makes the process expensive and time consuming. Additionally, the cold 'rolling'step prohibits the plating of parts that are already formed or shaped, and consequently the process is limited to use on flat, 11nfinished materials.

Another method of combining refractory alloys and base metals provides for heating the plated metals to a temperature at or above the melting point of the eutectic of the metals in contact. This method is described in U. S. Patent No. 2,156,262 to .et al., and, :as explainecl, has an advantage over the cold rolled process in that formed articles can be coated and do :not :lose their form even though heated at ,or above the melting point of the eutectic.

tion to prov de .a simplified method of making a nickelchromium diffusion alloy whereinthe heat treatment is carried out at temperatures substantially below the melting points of the metals and wherein no cold-rolling step is required.

Other objects and advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

Figure l is a partial sectional view showing laminates of chromium and nickel deposited on a base metal;

Figure 2 is a partial sectional view showing the condition of the laminates of Figure 1 after heat treating; and

Figure 3 is a diagram comparing the growth of molybdenum-nickel and molybdenum-chromium diffusion zones with time at various temperatures.

Referring now .to the drawing, there :is shown in Figure 1 a cross-sectional view of deposited laminates of chromium and nickel as they would appear afterplating and before heat treating. Various numbers of layers of laminates can be deposited, depending upon the final desired thickness, and the thickness of the various layers .can be varied to determine the desired ratio of nickel to chromium in the alloy. By way of example, if an '80 nickel-20 chromium (by weight) diffusion alloy is desired, five (5) layers of each may be deposited, alternately, the nickel layers each being .00075 inch thick and the chromium layers each being .00025 inch thick.

As shown in Figure 3 of the drawing, nickel diffuses and alloys faster with molybdenum than does chromium.

Also nickel forms hard, brittle compounds with molyb-' denum, whereas chromium does not. For these reasons, chromium is the better of the two electrodeposits for direct contact with molybdenum at high temperatures,

and it is desirable to plate a substantial thickness (about .0015 inch) 'of chromium on the molybdenum base metal before depositing the laminates thereon.

By way of example, a specimen to ,be plated with nickel-chromium laminates is first pretreated by' degrea'sing in an alkali, pickled in nitric acid solution; and then etched in a chromic acid-sodium dichromate solution. 'A .0015 inch thick chromium plate is then deposited using the following bath formulation and operating conditions:

Formulation:

250 g./1. chromic acid 2.5 ,g./l. sulfuric acid Operating conditions:

Cathode current density 750 amps./sq."ft. Anodes Chemical lead sheet. Bath temperature 185 F.

After plating with a .0015 inch layer of chromium, which requires about 60 minutes of plating time, the specimen is then etched in 37 percent hydrochloric acid, and then the following procedure is followed to deposit alternate layers of chromium and nickel:

(1) Soak for 1 minute in 180 F. distilled water made acid with sulfuric acid to pH 1. a

-(2) Place in a chromium plating bath, as described above, and plate with a .00025 inch of chromium.

(3) Rinse in water.

(4) Apply a nickel-strike plate in a nickel sulfate bath having the following formulation and operating conditions:

Formulation 450 g./l. nickel sulfate 50 g./l. sulfuric acid Operating conditions:

Cathode current density 100 amps/sq. ft. Anodes Carbon rods. Plating time 1 minute. Bath temperature 105 F.

(5)Applynickel plate .00075 inch thick from a sulfamate-nickel bath having the following formulation and operating conditions:

Formulation:

450 g./l.' nickel sulfamate 30 g./l. boric acid 0.375 g./l. antipit agent (for example, SNAP, a product of Barrett Chemical Products, Shelton,

Conn.) Bath pH-4.0 Operating conditions:

Cathode current density 80 amps/sq. ft. Anodes Cast nickel. Plating time 11.2 minutes. Bath temperature 145 F.

Agitate specimen.

metallographic study of several heat-treated specimens established that all the nickel and chromium layers were alloyed after 4 hours heating in air at a temperature of 980 C., as the alloy was not attacked by the oxalic acid electrolytic etch for chromium or the 50:50 lactic- V nitric acid chemical etchant for nickel. The ,difiusion alloy was nonmagnetic, indicating that all the nickel had absorbed at least 6 percent chromium as a nickelchromium alloy transforms from a magnetic to a nonmagnetic state at 6 percent chromium, as the chromium content increases. Metallographic evidence of the diffusion alloy shown in Figure 2 of the drawing indicated a i that the alloy was homogeneous and also Knoop hardness measurements across an etched section were between 170 and 185, confirming the metallographic evidence for homogeneity.

In an experiment'conducted by the inventor, molybdenum rods, inch in diameter and 2 inches long, were heat treated for 2 hours at 980 C. in hydrogen, plated with .0015 inch of chromium' plus alternate layers of nickel (.0075 inch) and chromium (.00025 inch), as explained above, until a total coating thickness of .0075

inch was applied. The nickel. and chromium were then 1 diffused by heating for 24 hours at 980 C. in hydrogen. in air for hours at 1796 F. which caused no blistering or pin holing in the protective coating visible at IOXmagnification. The scale was uniform in appearance and was adherent.

Although electroplates of nickel plate or chromium have been made that have survived simulated service tests, the advantage of this invention is that the nickelchromium diffusion alloy is more oxidation-resistant than either nickel or chromium and it also appeared that the coating probablywould be satisfactory for a much longer period of exposure.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings." It is therefore to be understood that the invention may be practiced otherwise than as specifically described.

What is claimed is: I 1. A process for coating molybdenum with a difiusio alloy which consists of depositing sequentially a layer of chromium about .00025 inch thick and a layer of nickel about .00075 inch thick, repeating said sequence of depositions a plurality of times, and then heating the coated molybdenum for 4 hours at a temperature above 980 C. and substantially below the melting point of the eutectic of the metals.

2. A process for coating molybdenum with a diffusion alloy which consists of the steps of first electroplating. a .0015 inch thick layer of chromium on said molyhdenum; next depositing sequentially a layer of chromium about .00025 inch thick, a nickel strike plate froma nickelsulfate bath, and a layer of nickel about .00075 inch thick by plating from a nickel sulfamate bath; re- 'peating said sequence of depositions a plurality of times,

and then heating said coated molybdenum about 4 hours at a temperature between 980 C. and a temperature. substantially below the melting point of the eutectic of the metals.

References Cited in the file of this patent UNITED STATES PATENTS 1,608,694 Cain Nov. 30, 1926 1,746,987 Bennett Feb. 11, 1930 1,792,082 Fink et a1 Feb. 10, 1931 2,402,834 Nachtman June 25, 1946 2,555,372 Ramage June 5, 1951 2,697,130 Korbelak Dec. 14, 1954 2,739,107 Ricks Mar. 20, 1956 2,772,227 Quaely et al Nov. 27, 1956' The diffusion alloy was then evaluated by heating 

1. A PROCESS FOR COATING MOLYBDENUM WITH A DIFFUSION ALLOY WHICH CONSISTS OF DEPOSITING SEQUENTLY A LAYER OF CHROMIUM ABOUT .00025 INCH THICK AND A LAYER OF NICKEL ABOUT .00075 INCH THICK, REPEATING SAID SEQUENCE OF DEPOSITIONS A PLURALITY OF TIMES, AND THEN HEATING THE COATED MOLYBDENUM FOR 4 HOURS AT A TEMPERATURE ABOVE 980*C. AND SUBSTANTIALLY BELOW THE MELTING POINT OF THE EUTECTIC OF THE METALS. 